1. Field of the Invention
This invention relates generally to the field of surface modification by ion implantation and particularly to ion implantation in irregularly shaped objects.
2. Description of Related Art
Ion implantation has been used to change the surface properties of a variety of materials for many purposes. Ion implantation hardens many materials and can increase the lifetime of friction-bearing parts. Ion implantation can improve corrosion resistance. Ion implantation can improve the lifetime of cutting tools, metal parts that operate in salt water or other corrosive environments, biological implants, and machine parts. Metals, ceramics, and plastics are among the types of materials it is desirable to implant. A general discussion of the benefits and techniques of ion implantation can be found in an article by S. Picraux, et. al., "Ion Implantation of Surfaces", Scientific American, 252 (3): 102-113, 1985. More technical discussion can be found in Handbook of Ion Implantation Technology, J. C. Ziegler, ed.; Elsevier Science Publishers B. V. (Amsterdam: 1992).
Ion implantation has typically been achieved by generating ions in a confined volume, either from a gas, by sputtering, or by vaporization of a solid, and then extracting them in a beam by applying an electrical potential gradient. A major part of the cost of building conventional ion implantation equipment is associated with extracting and focusing the ion beam on the target to be implanted. At the point the beam hits the target, the ions must match an energy and density specification that results in the desired implantation characteristics. Typically, in an attempt to uniformly implant a target surface, the beam is rastered back and forth across the surface which faces the beam. If many sides of the target are to be implanted, it is typically attached to a mobile stage that supports rotation of the target in the ion beam allowing all desired surfaces to be implanted. This process is slow. It is also difficult to prevent shadowing of the beam if the target contains uneven surfaces, as for example, in a screw or a nut. Thus efficient use of ion beams to implant targets is limited to simple shapes. An additional problem in use of ion beams for implantation is the complex stage that is necessary to rotate the target in the beam.
Recently John R. Conrad obtained patent U.S. Pat. No. 4,764,394 (University of Wisconsin Alumni Association) for a method and apparatus for implanting three-dimensional materials by achieving implantation from all sides of the target simultaneously. Conrad places the target in a plasma chamber, then applies a high negative potential pulse to the target relative to the walls of the chamber (pulsed target system). The high voltage pulse causes the ions to accelerate into the target over all exposed surfaces. The target may have any of a variety of shapes (FIG. 1).
There are however several difficulties encountered when applying a large, pulsed, negative potential to the target. Because, during the rise time of a pulse, the voltage varies between zero and the maximum potential applied, the energy of individual ions approaching the target varies as a function of time. As a result, the penetration of the ions into the target is spread as a function of time (see FIG. 1B, rise time of pulse). That is, the ions have a broad implant profile. This is undesirable for applications in which a uniform or controlled deposition layer is needed, such as in several oxygen implantation applications.
A second problem with the pulsed target system is that because the plasma source is always on, considerable secondary emission electrons, created when the ions interact with the chamber walls, are introduced into the chamber. This affects the plasma and the total current in the circuit used to apply voltage to the target. (see FIG. 1A) The total current (I.sub.T) in that circuit is the sum of the current resulting from the current drawn by the object (I.sub.impact) and the secondary emission electrons leaving the target (I.sub.e). That is, I.sub.T =I.sub.impact +I.sub.e. As I.sub.impact increases, the switch needed to control the applied high potential becomes more difficult to design. For a large target, I.sub.e by itself can be large enough to require a specially-designed, expensive electronic switch. A complex circuit is then required, capable of switching a high-current high-voltage power source (FIG. 1C). An improved pulse modulator switch, invented specifically for this application, is disclosed in U.S. Pat. No. 5,212,425, issued May 18, 1993 to D. M. Goebel and J. N. Matossian and assigned to Hughes Aircraft Company. This modulator switch has increased duty factor and power handling capabilities over that used by Conrad. However it would be preferable to eliminate the need for such a complex switch.
Another problem associated with a plasma source that is always on is that heat loading on the target due to plasma bombardment and radiation heating can be high. In U.S. Pat. No. 5,218,179 issued to J. N. Matossian and D. M. Goebel on Jun. 8, 1993 and assigned to Hughes Aircraft Company, it is calculated that using the plasma-production process disclosed in Conrad's patent, as much as 1 W/cm.sup.2 of target heating is produced for large-scale targets such as dies and tools used in manufacturing automobiles. In their patent, Matossian and Goebel disclose a novel arrangement for ion source implantation wherein the plasma is generated in a chamber which is separate from, and opens into, an implantation chamber. However, it would be preferable to avoid the need for complex geometries that separate the plasma generation volume from the implantation volume.
Matossian and Goebel, in their patent '179 address yet another difficulty with Conrad's basic plasma-source-ion-implantation technique. In order to obtain high plasma densities, many filaments must be used and heated to temperatures on the order of 2000.degree. C. At this temperature, so much evaporation of the filament material occurs that the target would be coated unless a complicated baffling was constructed. Matossian and Goebel disclose use of a separate plasma generating chamber in which line-of-sight communication between the plasma generating source and the target is minimized, thereby minimizing the transfer of filament material to the target. A clean plasma source would provide a better solution to the problem of filament contamination by evaporation or sputtering.
Where more than one ion species is desired for implantation, as in the case of multi-ion implantation, the ion production of each ion specie cannot be selectively controlled using the plasma ion implantation system of Conrad. Matossian and Goebel disclose in their patent '179, that use of two or more separate plasma chambers enables them to generate plasmas of different ion species which diffuse into, and mix in, the implantation chamber. More desirable would be the capability of producing mixed ion species without use of specially designed, geometrically-complex, separated chambers to generate the plasma for each ion desired.
A further problem associated with maintaining the plasma on constantly during implantation is the introduction of ionizing x-rays into the plasma. Ionizing x-rays give rise to yet more secondary electrons. The plasma density varies with the presence of secondary electrons and becomes difficult to control resulting in irregular depth of ion implantation.
A sixth difficulty with the pulsed target system is that electrical breakdown between the plasma and the target occurs at corners on the target. When no plasma is present, the voltage applied to the target is between the target and the walls of the plasma chamber. However when the plasma is on, the entire voltage drops across the small distance between the plasma boundary and the surface of the target. Thus the useful applied voltage is limited to the breakdown voltage (V.sub.BD) across the small gap that separates the plasma boundary and the target. The problem is worst near portions of the target with small radius of curvature yielding the problematic result of degrading sharp corners and morphological detail on the target.
These problems with the current methods of ion implantation limit the usefulness of the technique to small objects without a high level of morphological detail.
A device in which large targets of varying shapes could be implanted with ions, of one or more species, to uniform depth, without overheating, would be extremely desirable. It would be additionally beneficial to be able to pulse the implantation voltage with a simple switch rather than a high power switch. A further advantage would be the ability to control or minimize the effect of secondary electrons on the plasma density so that low density plasmas could be used. It would be even more advantageous to minimize voltage breakdown thus preserving the morphological detail on the target.